The new generation projection lithography for large-scale production of integrated circuits IC with structure sizes of 10 nm or less is based on the use of EUV radiation in the range of 13.5+/−0.135 nm corresponding to effective reflection of multilayer Mo/Si mirrors. The control of the IC to be defect-free is one of the most important metrological processes of modern nanolithography. The general trend in lithographic production is a shift from IC inspection, which is extremely time-consuming and costly in large-scale production, to the analysis of lithographic masks. In the case of mask defects they are projected onto a silicon substrate with a photoresist, resulting in the appearance of defects on the printed chips. The mask in EUV lithography is a Mo/Si mirror, on top of which a topological pattern is applied from a material that absorbs radiation at a wavelength of 13.5 nm. The most efficient method for the process of mask inspection is carried out at the same wavelength for actinic radiation, that is, radiation, whose wavelength coincides with the working wavelength of the lithography the so-called Actinis Inspection. Such scanning by radiation with a wavelength of 13.5 nm allows the detection of defects with a resolution better than 10 nm.
Thus, the control of defect-free lithographic masks in the process of their production and during the entire period of operation is one of the key challenges for EUV lithography while the creation of a device for the diagnosis of lithographic masks and its key element—a high-brightness actinic source—is a priority for the development of EUV lithography. For these purposes, it is required to develop a relatively compact and economical device on the basis of an EUV source with high brightness radiation B13.5≥100 W/mm2·sr in the spectral band of 13.5+/−0.135 nm and with a small value of etendue G=S·Ω≤10−3 mm2sr, where S is the source area in mm2, Ω the solid angle of the output EUV radiation in steradian.
The radiation sources for EUV lithography are using Sn— plasma generated by a powerful laser system including CO2 lasers. Such sources have the power of EUV radiation exceeding by several orders of magnitude the level of power required for the inspection of EUV masks. Therefore, their usage for mask inspection is inadequate due to the excessive complexity and cost. In this regard, there is a need for other approaches to the creation of high-brightness EUV sources for actinic inspection of EUV masks.
In accordance with one of the approaches, known from U.S. Pat. No. 7,307,375, issued on 12 Nov. 2007, in a high-brightness source of EUV radiation, a pulsed inductive discharge is used to create an electrodeless Z-pinch in gas, in particular, Xe. The device includes a pulsed power system connected to the primary winding coil of the magnetic core that surrounds part of the discharge zone. In this case, the Z-pinch is formed inside an insulating ceramic SiC sleeve with an opening diameter of about 3 mm. This results in sufficiently strong erosion and means the sleeve requires frequent periodic replacement. The source is characterized by simplicity, compactness and relatively low cost. However, the size of the radiating plasma is relatively large, and the maximum reported brightness of the source ˜10 W/mm2 sr is lower than that required for a number of applications, including lithographic mask inspection.
This drawback is largely avoided in the device according to the patent application US20150076359, issued on 19 Mar. 2015 which also includes a new method for generating EUV radiation from laser produced plasma. In the embodiment of this invention, the target material is xenon, which is frozen onto the surface of a rotating cylinder cooled by liquid nitrogen. The laser plasma radiation collected by the collector mirror is directed to an intermediate focus. The device and the method allow the achievement of a small size of plasma emitting in the EUV range, a greater brightness of the radiation source up to 80 W/mm2·sr in the absence of any contamination of the optics. The disadvantages of this method include insufficiently high efficiency of the plasma-forming target material and the high cost of xenon which requires a complex system for its recirculation.
From the U.S. Pat. No. 8,344,339, issued on 1 Mar. 2012, a known device for the generation of EUV radiation from laser produced plasma including: a vacuum chamber, which houses a rotating rod made of plasma-forming target material, an input window for the laser beam focused in the interaction zone of the laser beam and target, and an EUV beam generated from the laser-produced plasma exiting an output window towards the optical collector. The device and the method of generation of EUV radiation are characterized by the fact that tin Sn is used as the most effective plasma-forming target material and the rod, in addition to rotation, also performs reciprocating axial movements. However, these devices and the method have a number of disadvantages, which include the non-reproducibility of the profile of the solid surface of the target from pulse to pulse during long-term continuous operation of the device, which affects the stability of the output characteristics of the short-wavelength radiation source. The complexity of the design is another disadvantage, since complex movements of the target assembly and its periodic replacement are required. During production of EUV radiation, debris particles are produced as a by-product, which can degrade the optics surface. The level of debris produced in this source is too high and that severely limits the possibilities of its application.
The debris, generated as a by-product of the plasma during the radiation source operation, can be in the form of high-energy ions, neutral atoms and clusters of target material.
The magnetic mitigation technique, disclosed for example in U.S. Pat. No. 8,519,366, issued on 27 Aug. 2013, is arranged to apply a magnetic field so that at least charged debris particles are mitigated. In this patent the debris mitigation system for use in a source for EUV radiation and/or X-rays, includes a rotatable foil trap and gas inlets for the supply of buffer gas to the foil trap so that neutral atoms and clusters of target material are effectively mitigated.
Another debris mitigating technique, known from U.S. Pat. No. 7,302,043, issued on 27 Nov. 2007, is arranged to apply a rotating shutter assembly configured to permit the passage of short-wavelength radiation through at least one aperture during the first period of rotation, and to thereafter rotate the shutter to obstruct passage of the debris through at least one aperture during the second period of rotation.
However, the complexity of using these debris-mitigating techniques in a compact radiation source means that technically they are too difficult to implement.